A porcine isolate of streptococcus mutans transferred tetracycline (Tc) resistance, by conjugation, to human strains of S. faecalis and S. mutans. A 12 megadalton plasmid was isolated from the transconjugant clones but not from the donor strain. Four of 16 streptococcal isolates obtained from the oral cavities of human patients were also able to transfer Tc resistance to S. faecalis. A second type of conjugative plasmid, pAM beta 1, mediating resistance to erythromycin and lincomycin, readily undergoes extensive deletions following transfer to new streptococcal host strains. These deletions, dependent upon the host in which they occur, involve-different regions of the parent molecule. Preliminary results suggest that several functions, such as transmissibility, replication and resistance, may be altered by the deletions. These data indicate that it will be possible to construct a physical and genetic map of plasmid pAM beta 1. Pentoses are fermented by very few strains of S. lactis. Strain DR1253, however, is able to utilize three pentoses, L-arabinose, D-xylose and D-ribose. The ability of this strain to metabolize D-xylose appears to be plasmid-mediated. The metabolic pathways by which L-arabinose and D-xylose are metabolized, via pentose isomerase and pentulokinase activities, are inducible in S. lactis and appear to be independently regulated.